Liquid ink printers of the type often referred to as drop-on-demand, such as piezoelectric, acoustic, phase change wax-based or thermal, employ at least one printhead from which droplets of ink are directed towards a recording sheet. Within the printhead, the ink is contained in a plurality of channels. Power pulses cause the droplets of ink to be expelled as required from orifices or nozzles at the end of the channels.
Liquid ink printers, including ink jet printers, deposit black and/or colored liquid inks which tend to spread when the ink is deposited on paper as a drop, spot, or dot. A problem of liquid ink printers is that the liquid inks used have a finite drying time, which tends to be somewhat longer than desired. Bleeding tends to occur when the drops are placed next to each other in a consecutive order or in a cluster of dots within a short time. Bleeding, spreading, and feathering causes print quality degradation including color shift, reduction in edge sharpness and solid area mottle which includes density variations in the areas due to puddling of inks.
Intercolor bleeding is a well-known problem that occurs when ink from one color area bleeds into or blends with ink from another color area. In particular, when an abrupt change in one or more separations occurs, with ink of different colors on each side of the interface or edge, the dye may diffuse across the edge, generally following paper fibers, and resulting in a ragged, at best, or badly blurred, at worst, edge. Intercolor bleeding is pronounced where an area of black ink (relatively slow drying) adjoins an area of color ink (relatively fast drying); however, intercolor bleeding can occur at the interface between areas of any color inks that dry slowly enough to mix before drying.
Various methods have been proposed to increase edge sharpness and to reduce intercolor bleeding. At least three general approaches have been used to provide acceptable image quality in the past: (1) employing surface coatings to absorb or polymerize the ink, preventing lateral diffusion; (2) limiting maximum drop count, either globally through color conversion tables or locally through image processing; and (3) tailoring ink formulations. The latter category includes so-called fast-dry inks, which penetrate the paper rapidly, giving a relatively “dry” surface when the next separation is printed, and Hewlett-Packard's specially formulated carbon black ink which precipitates out of solution immediately on contact with any of the colored inks with which it is used. While all of the proposed methods reduce intercolor bleeding to some degree, they all have one or more drawbacks that affect printer performance and/or image quality.
In the office market specific paper requirements are unacceptable, particularly for everyday use. Additionally, ink design involves many tradeoffs which don't always allow one to optimize productivity, optical density and bleed simultaneously. For example, using a fast dry ink in place of a slow drying black ink often results in a reduced quality of black reproduction as most fast drying black inks generally available have lower image quality than slow drying black inks. Additionally, fast drying black inks typically result in fuzzy edges in black areas next to non-printed areas. Using fast drying black ink at an interface and slow drying black ink for interior regions can eliminate lower image quality associated with fast drying black inks, but increases the cost and complexity of printer design by requiring a fifth ink in addition to the cyan, magenta, yellow and slow drying black ink.
For the photo-finishing market coated papers are standard. Nonetheless, ink reduction at edges can be an important component of a system strategy to limit coating thickness (cost) or color-color registration requirements (latitude). Multiple-drop per pixel printing sometimes needs a large number of drops to provide maximum optical density (for black) and saturation (for colors). At the same time, such large numbers of drops virtually ensure bleed at color/color edges on plain paper and even on some coated media. The problem, then, is to ensure that at edges the number of drops is reduced, while elsewhere it remains as specified by the color conversion and error diffusion or halftoning processes.
Prior solutions in similar situations appear in U.S. Pat. No. 4,930,018 (to Chan) and U.S. Pat. No. 5,635,967 (to Klassen). The former applies to multiple drop per pixel, but uses reduced resolution and limits the drop count throughout the image through a modified error diffusion. It accepts as input a continuous tone image and generates a drop count for each of several dye loadings at each pixel. The latter, on the other hand, accepts a single drop per separation per pixel image, reducing average drop count at edges. Additionally, in a series of closely related U.S. patents including U.S. Pat. Nos. 6,361,144 and 6,290,330 (to Torpey); U.S. Pat. No. 6,270,186 (to Smith); and U.S. Pat. No. 6,183,062 (to Curtis) there is taught the reduction of intercolor bleeding at the interface between regions as well as the reduction of bleeding at edges of printed/non-printed regions by modifying the pixels within a border region along the edge or interface to reduce the amount of ink deposited within the border region.
To address the problem of intercolor bleeding, there is taught herein a method of reducing intercolor bleeding by finding edges in an image and reducing the number of drops printed at those edges that would otherwise be prone to (likely to experience) intercolor bleeding. In accordance with the teachings herein, there is also disclosed a method employing small look-up tables to find edges within separations and to determine an amount to reduce ink on both sides of the edge, e.g., determine reduction of ink, or determine maximum amount of ink to be deposited at a given edge. The table based edge finding eliminates the need for special image processing or digital front end (DFE)-based tags. An additional benefit arises in that the method may be employed to generate a signal that may be used to determine where to erode cyan, magenta, and/or yellow separations to avoid color fringing at the edges of four color black (areas wherein process black is mixed with black ink). The present teachings find applicability in both single (i.e., fixed volume) drop per pixel ink deposition and multidrop per pixel (as well as variable volume drops) printing. The look-up table based edge finding and color modification is quick and can be implemented in hardware/software. Additionally, the teachings herein can operate in the drop count domain rather than continuous tone.
In accordance with one embodiment disclosed herein, there is taught a method of processing image data, comprising: receiving a plurality of pixels, each pixel having a plurality of separation values; analyzing the image data to identify pixels located at an edge; determining a coverage value from the separation values for the identified edge pixels; determining a reduction factor for the identified edge pixels based on the coverage value; and determining reduced separation values for the identified edge pixels based on the reduction factor.
In accordance with one embodiment disclosed herein, there is taught a system for reducing intercolor bleeding. The system includes an input data register receiving a plurality of pixels from the image data, each pixel having a plurality of separation values; a detection circuit analyzing the separation values of the plurality of pixels and generating a detection signal indicating whether an edge was detected; a summer receiving the separation values of the plurality of pixels from the input data register and compiling a coverage value from the received separation values; a lookup table generating a reduction factor based on the coverage value; a selector receiving the reduction factor and the detection signal, the selector generating a multiplier signal based on one of the reduction factor and a default value in response to the detection signal; and a reduction circuit generating reduced separation values for the plurality of pixels in response to the reduction signal.